Non-Uniform Data Complexity of Query Answering in the Presence of Weakly Guarded TGDs

Ontology-based data access (OBDA) is a new and promising paradigm in database and logics research and offers the added benefit that detailed world knowledge about a subject can be embodied in an ontology, in addition to the classical instance database. Description logics are a prime candidate for expressive and efficient ontology languages, and complexity analysis of the accompanying reasoning tasks has recently been enriched by the non-uniform approach. Another formalism used in OBDA are existential rules, which in the form of the new Datalog± family of languages meet increasing interest. Being computationally challenging in general, subclasses like weakly guarded TGDs have been identified which entail ExpTime data complexity. The present thesis combines the novel non-uniform approach to examine data complexity of weakly guarded rules, and confirms a rich and non-dichotomic structure below ExpTime by presenting rule sets for NP, PSpace and all levels Σpi of the polynomial hierarchy.